Optical fibers including a core and a cladding are well known as a light-transmitting medium. Among them, in recent years, attention has been paid to PCFs as promising optical fibers which allow communications in a novel wavelength band that have not been realized with conventional optical fibers and offer enhanced communication speed with reduced costs. The PCF includes a solid or hollow core which is provided in the center of the fiber and a cladding which surrounds the core and has a plurality of pores extending along the core. The PCF encloses light in the core surrounded by the cladding to transmit the light and controls the dispersion of light wavelength without restraint by changing the size of or the interval between the pores.
The PCF may be formed by: filling a cylindrical support tube with a plurality of capillaries for forming the cladding such that the capillaries are closest-packed therein when viewed in section; providing a core rod for forming the solid core or core space for forming the hollow core at the center axis position to obtain a preform; and thinning the preform by heating and drawing. This is called a stack-and-draw technique. This technique is a universal one because it allows relatively easy manufacture of PCFs having a large number of pores.
According to the stack-and-draw technique, however, the cylindrical, ultrasmall-diameter capillaries freely roll within the large-diameter support tube along the circumferential direction of the support tube. Since the positions of the capillaries are not fixed within the support tube while packing the capillaries into the support tube, there is a problem of great difficulty in closest-packing the cylindrical capillaries in the cylindrical support tube. For example, as shown in FIG. 6, the support tube 3 is not completely filled with the capillaries 1 and the capillaries 1 are spaced from each other or the arrangement of the capillaries 1 is disturbed. As a result, the center axis of the core rod 2 for forming the core may be misaligned from the center axis of the support tube 3. In such a case, about 70% of the capillaries 1 functions as photonic crystal after drawing. Therefore, JP2002-97034A has disclosed a technique of providing the inner wall surface of the support tube 3 with a substantially regular hexagonal cross-sectional shape.
Giving the substantially regular hexagonal cross-sectional shape to the inner wall surface of the support tube 3 is advantageous for the closest packing of the capillaries 1. However, to obtain the inner wall surface of the support tube 3 having the substantially regular hexagonal cross-sectional shape, there is no other way than to use a common grinding technique. Therefore, it is difficult to grind a columnar rod into a cylinder with great precision to give the inner wall surface thereof the substantially regular hexagonal cross-sectional shape. Further, the length of the support tube 3 to be obtained is limited to as short as about 200 mm. Therefore, this technique is not suitable for fabricating a long PCF. Thus, the technique of forming the support tube 3 whose inner wall surface has the substantially regular hexagonal cross-sectional shape is unrealistic from the technical and economic viewpoints.
On the other hand, for optical fiber sensors and coherent optical communications making use of polarization and coherence of light, polarization maintaining optical fibers have been used which maintain the polarization of transmitting light and have high polarization stability. Taking the wavelength dispersion property as an advantage, use of the above-described PCFs as the polarization maintaining PCFs has been studied. To fabricate the polarization maintaining PCFs, a twist should be added to the core or the arrangement of the pores around the core. For example, the core may be oval or rectangular when viewed in section or some of the pores adjacent to the core may be given with different diameter from the others.
As to the polarization maintaining PCFs which place great importance on axiality, the closest packing of the capillaries in the support tube is still extremely difficult. Therefore, it is difficult to fabricate the PCF in which all the pores in the cladding function as photonic crystal.